1. Field of the Invention
The present invention relates to an image forming apparatus implemented by a distributed control system including a plurality of CPU groups having a hierarchical structure.
2. Description of the Related Art
Centralized control using one CPU is performed for printer device control of an image forming apparatus using an electrophotographic system. An increasing CPU load due to centralized control requires a higher performance CPU. In addition, as the load of the printer device rises, communication cables (a bundle of communication lines) need to be laid from a CPU board to distant load driver units and many long communication cables are indispensable. To solve these problems, a control form is receiving a great deal of attention, in which control modules that build an electrophotographic system are assigned to sub-CPUs.
Examples of the construction of a control system by distributing respective partial module control functions to a plurality of CPUs have been proposed in several control equipment product fields other than copying machines. For example, Japanese Patent Laid-Open No. 2000-071819 has proposed a technique of hierarchically locating functional modules in a vehicle and performing distributed control. Japanese Patent Laid-Open No. 2006-171960 has proposed a technique of applying a similar hierarchical control structure to robot/automation equipment. These sub-CPUs require a communication unit to operate them as a system as a whole. According to Japanese Patent Laid-Open No. 2006-171960, separate communication networks are constructed for respective hierarchical layers in a control network for performing communication between functional modules. By distributing loads, a more stable control network can be built.
However, these conventional techniques suffer the following problems. When this system configuration is applied to distributed control of an image forming apparatus, a plurality of CPUs operate at the same operating frequency. The peak value of the frequency spectrum of a clock signal rises, increasing radiation noise. The radiation noise may be suppressed by designing different operating frequencies for respective CPUs. However, the operating frequency needs to be determined for each image forming apparatus model. This leads to a larger number of design steps and higher design cost. Further, factory management of CPUs having different operating frequencies is essential and boosts the manufacturing cost.